if it is narrowly tailored to further a compelling government interest and no less

restrictive alternative would serve that interest.  We acknowledge that use of filtering

software furthers public libraries' legitimate interests in preventing patrons from

accessing visual depictions of obscenity, child pornography, or in the case of minors,

material harmful to minors.  Moreover, use of filters also helps prevent patrons from

being unwillingly exposed to patently offensive, sexually explicit content on the Internet. 

We are sympathetic to the position of the government, believing that it would be

desirable if there were a means to ensure that public library patrons could share in the

informational bonanza of the Internet while being insulated from materials that meet

CIPA's definitions, that is, visual depictions that are obscene, child pornography, or in

the case of minors, harmful to minors.  Unfortunately this outcome, devoutly to be

wished, is not available in this less than best of all possible worlds.  No category

definition used by the blocking programs is identical to the legal definitions of obscenity,

child pornography, or material harmful to minors, and, at all events, filtering programs

fail to block access to a substantial amount of content on the Internet that falls into the

categories defined by CIPA.  As will appear, we credit the testimony of plaintiffs' expert

Dr. Geoffrey Nunberg that the blocking software is (at least for the foreseeable future)

incapable of effectively blocking the majority of materials in the categories defined by

CIPA without overblocking a substantial amount of materials.  Nunberg's analysis was

supported by extensive record evidence.  As noted above, this inability to prevent both

substantial amounts of underblocking and overblocking stems from several sources,

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